German Patent Application No. DE 102 004 011 203 describes various examples of methods for mounting semiconductor chips and corresponding semiconductor chip systems of which three examples will be discussed in greater detail in the following text.
FIG. 7 shows a first example for a method of mounting semiconductor chips and a corresponding semiconductor chip system in a cross-sectional view.
In FIG. 7, reference numeral 100 denotes a TO8 base produced from Kovar, for example. Reference numeral 5 is a micromechanical silicon pressure sensor chip having piezoresistive transducer elements 51, which are accommodated on a diaphragm 55. To produce diaphragm 55, a cavity 58 is introduced on the back of respective silicon pressure sensor chip 5, for instance by anisotropic etching, e.g., using KOH or TMAH. Alternatively, diaphragm 55 may also be produced by trench-etching.
Sensor chip 5 may be made up of a pure resistance bridge having piezoresistive resistors, or it may be combined with an evaluation circuit, which is integrated together with the piezoresistors in a semiconductor process. A glass base 140 made of sodium-containing glass, which is anodically bonded to the back of chip 5, is used to reduce mechanical stress caused by solder or adhesive 70 with whose aid glass base 140 is mounted on TO8 base 100. Reference numeral 53 in FIG. 7 denotes a bond pad of an integrated circuit 52 (not further shown), the bond pad being connected via a bonding wire 60 to an electrical connection device 130, which in turn is insulated from TO8 base 100 by an insulating layer 131. Glass base 140 has a through hole 141, which connects cavity 58 to externally prevailing pressure P, via a through hole 101 of TO8 base 100 and a connection device 120 affixed thereto. The configuration shown in FIG. 7 is usually also welded to a metal cap (not shown) so as to form a tight seal.
However, such a configuration has the disadvantage that it is cumbersome and that problems frequently arise with the hermetic enclosing of sensor chip 5, for instance because of leaky welding seams or the like. Since the TO8 housing and the silicon have different thermal coefficients of expansion, temperature changes cause mechanical stresses, which are measured as interference signals by piezoresistors.
FIG. 8 shows a second example for a method of mounting semiconductor chips and a corresponding semiconductor chip system in a cross-sectional view.
In this second example sensor chip 5 is bonded above a glass base 140′, which has no through hole, to a substrate 1 made of ceramic or plastic, and passivated using a gel 2 to protect it from environmental influences. Additionally provided above the chip system on substrate 1 is a protective cap 13, which has a through hole 15 for pressure P to be applied. Glass base 140′ also has no through hole in this example, since pressure P is applied from the other side.
When such a gel 2 is used, the maximum pressure is disadvantageously determined by gel 2, since gas diffuses into gel 2 and, if there is a sudden reduction in pressure, gas bubbles are created in gel 2, which destroy gel 2.
FIG. 9 shows a third example for a method of mounting semiconductor chips, and a corresponding semiconductor chip system in a cross-sectional view.
In this example, sensor chip 5′ is a surface-micromechanical sensor chip, which was produced according to the method described in German Patent Application No. DE 100 32 579 A1, for example, and has an integrated cavity 58′ above a diaphragm region 55′.
For mounting, bond pads 53 of sensor chip 5′ are soldered onto bond pads (not shown) of the substrate in a mounting region, using a solder or bonding connection such as solder balls 26; the substrate is part of a premold housing 10 made of plastic from which a leadframe 8 formed therein projects on the side. Premold housing 10 has a recess 11, next to which sensor chip 5′ is mounted in flip-chip technique so as to protrude.
The minimum clearance of the contact regions of leadframe 8 in the mounting region of sensor chip 5′ is usually greater than the minimum clearance between bond pads 53 on sensor chip 5′. However, since only a few bond pads 53, such as four pieces for connecting a Wheatstone bridge, are required on sensor chip 5′, they may be placed as far from one another as necessary.
In addition, the mounting region has underfilling 28 made of an insulating plastic material, and edge K of recess 11, which lies between the mounting region and diaphragm region 55, is used as separation edge for underfilling 28 during the mounting process. Separation edge K ensures that underfilling 28 is unable to get into or under diaphragm region 55′. Diaphragm region 55′ of sensor chip 5′ thereby laterally projects next to the strip-shaped mounting region, so that the pressure medium is able to reach diaphragm region 55′ without hindrance.
In diaphragm region 55′, the surface of sensor chip 5′ is passivated by a layer (not shown), e.g., a nitride layer, which acts as a reliable medium protection. Underfilling 28 protects the mounting region of sensor chip 5′ from corrosion.
Finally, premold housing 10 has an annular sidewall region 10a on whose upper side a cover 20′ is provided, which has a through hole 15b for pressure P to be applied. Since sensor chip 5′ is distanced from the upper side of premold housing 10 by the flip-chip mounting on the side of the peripheral region opposite the mounting region, an uncomplicated transmission of applied pressure P to diaphragm region 55 is ensured.
Furthermore, cover 20′ has a pressure connection nipple 21, and an optional filter 22, which prevents particles or fluid media from gaining access to the interior of the sensor packaging, is able to be installed in through hole 15b. This may prevent the entry of water, for instance, which, when freezing, could forcibly dislodge and thereby destroy sensor chip 5′.
In this example, glass base 140, 140′ according to FIGS. 7 and 8, respectively, whose production entails considerable expense, may be omitted completely since the lateral projection of surface-micromechanical sensor chip 5′ next to the strip-shaped mounting region already allows the dissipation of the stresses that are produced by different temperature expansion coefficients of silicon and the connection via solder balls 26 and underfilling 28.